The present invention relates to porous activated carbon materials having controlled oxygen content and more specifically to highly porous activated carbon materials and methods for forming highly porous activated carbon materials having an oxygen content that is lower than that for carbon materials produced by conventional processes. The invention also relates to high power density energy storage devices comprising controlled oxygen content carbon-based electrodes.
Energy storage devices such as electric—also called electrochemical—double layer capacitors (EDLCs), a.k.a. supercapacitors or ultracapacitors may be used in many applications where a discrete power pulse is required. Such applications range from cell phones to electric/hybrid vehicles. An important characteristic of an ultracapacitor is the energy density that it can provide. The energy density of the device, which comprises one or more carbon electrode(s) separated by a porous separator and/or an organic or inorganic electrolyte, is largely determined by the properties of the carbon electrodes and, thus, by the properties of the carbon material used to form the electrodes.
Indeed, the performance of an energy storage device comprising carbon-based electrodes is largely determined by the physical and chemical properties of the carbon. Physical properties include surface area, pore size and pore size distribution, and pore structure, which includes such features as pore shape and interconnectivity. Chemical properties refer particularly to surface chemistry, which relates to the type and degree of surface functionalization.
Carbon electrodes suitable for incorporation into EDLCs are known. High performance carbon materials, which form the basis of such electrodes, can be made from natural and/or synthetic carbon precursors. For example, activated carbon can be made by heating a synthetic carbon precursor in an inert environment to a temperature sufficient to carbonize the precursor. During or following the process of carbonization, the carbon material can be activated. Activation can comprise physical activation or chemical activation.
Physical activation is performed by exposing the carbon material to steam or carbon dioxide (CO2) at elevated temperatures, typically about 800-1000° C. Activation can also be carried out by using an activating agent other than steam or CO2. Chemical activating agents such as phosphoric acid (H3PO4) or zinc chloride (ZnCl2) can be combined with the carbon material and then heated to a temperature ranging from about 500-900° C.
As an alternative to performing chemical activation post-carbonization, one or more chemical activating agents can be combined with a carbon precursor in a curing step prior to carbonization. In this context, curing typically comprises mixing a carbon precursor with a solution of an activating agent and then heating the mixture in air. In addition to phosphoric acid and zinc chloride, chemical activating agents may also include KOH, K2CO3, KCl, NaOH, Na2CO3, NaCl, AlCl3, MgCl2 and/or P2O5, etc.
In embodiments where a chemical activating agent is used, it is preferred to homogeneously distribute the chemical activating agent throughout the carbon precursor at a molecular level prior to curing the carbon precursor. This molecular level mixing prior to curing enables a homogeneous distribution of porosity after activating.
The activated carbon product can be washed in an acid or base solution and then with water to remove both the activating agent and any chemical species derived from reactions involving the activating agent. The activated carbon can then be dried and optionally ground to produce material comprising a homogeneous distribution of nanoscale pores. Activated carbon produced by this method offers significantly higher energy storage capacity in EDLCs compared to major commercial carbons.
Whether the carbon material is activated using physical or chemical activation, the incorporation of oxygen into the carbon, especially in the form of oxygen-containing surface functionalities, can adversely affect the properties of energy storage devices that comprise electrodes made from the activated carbon. For example, the presence of oxygen-containing surface functionalities can give rise to pseudocapacitance, increase the self-discharge or leakage rate, cause decomposition of the electrolyte, and/or cause a long term increase in resistance and deterioration of capacitance.
Oxygen functionalities can be introduced both during the curing step, where the mixture of carbon precursor and activating agent is oxidized at intermediate temperatures, and during the carbonization and activation steps, where the activating agent (e.g., steam or KOH) serves as an oxidation agent.
As a result of the potentially deleterious effects of incorporated oxygen, it can be advantageous to control and preferably minimize the oxygen content in activated carbon for use in energy storage devices such as EDLCs. Accordingly, it would be an advantage to provide a highly porous activated carbon material having a controlled oxygen content that can be used to form carbon-based electrodes that enable high energy density devices.
These and other aspects and advantages of the invention can be accomplished according to one embodiment wherein, after synthesis of an activated carbon material, a post-synthesis refining step comprises heating the activated carbon material in an inert or reducing environment at an elevated temperature. The inert or reducing environment is preferably substantially free of oxygen. According to a further embodiment, highly porous activated carbon having controlled oxygen content can be prepared by using a modified curing step. In the modified curing step, prior to the steps of carbonization and activation, a carbon precursor/chemical activating agent mixture is heated to an intermediate temperature in an environment that is substantially free of oxygen. The substantially oxygen-free environment can comprise one or more inert or reducing gases. A still further embodiment combines both approaches.
The activated carbon materials produced according to the present invention can be characterized by nanoporosity, high specific capacitance when incorporated into EDLCs, and controlled oxygen content. In one embodiment, the inventive activated carbon has a total oxygen content of less than 10 wt. %.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, as well as the appended claims.
It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed.